Hitherto, a fluorescence detection system in which a fluorochrome is used as a marker has been used broadly in biochemistry or molecular biology. By using the fluorescence detection system, gene arrangement, gene mutation polymorphism analysis, protein separation and identification, or the like can be evaluated and the system is used in development of medicine and the like.
As an evaluation method using the fluorescent marker described above, a method is frequently used in which biological compounds including protein or the like are distributed in gel by electrophoresis and the distribution of the biological compounds is obtained by using fluorescence detection. In the electrophoresis, electrodes are disposed in a solution such as a buffer solution, and a direct current flows through the electrodes to generate an electric field gradient in the solution. At this time, when there is protein, Deoxyribonucleic acid (DNA), or ribonucleic acid (RNA) with electric charges in the solution, biomolecules can be separated in such a manner that molecules with plus charges are drawn to a negative terminal, and molecules with minus charges are drawn to a positive terminal.
Two-dimensional electrophoresis as an example of the evaluation method using the electrophoresis is an evaluation method in which biomolecules in a gel are two-dimensionally distributed by combining two types of electrophoresis methods. This method is considered as the most efficient method for proteome analysis.
As a combination of the electrophoresis, for example, two methods are mainly used: “isoelectric focusing using a difference between isoelectric points of pieces of protein” in a first dimension and “SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis) performing separation by a molecular weight of protein” in a second dimension. Fluorochromes are given to the protein as the biomolecules separated in this manner before or after electrophoresis.
An image reading device has come into wide use in biochemistry or molecular biology, and in the image reading device, excitation light is applied to a gel support in which the biomolecules (protein) produced in the above-described manner are two-dimensionally distributed, intensity of generated fluorescence is obtained, and a fluorescence distribution (protein distribution) image is displayed based on the obtained fluorescence intensity.
As a method of holding the two-dimensional distribution of the biomolecules, a method is also performed in which the protein is separated in the gel, and then the separated protein is transferred from the gel to a membrane by using electrophoresis or capillary phenomenon, in addition to holding of the distribution in the gel. In that case, an image reading device can image fluorescence distribution on a transfer support which is the membrane, similarly to a case of reading an image by using the gel support.
Japanese Unexamined Patent Application Publication No. 2001-74656 (PTL 1) discloses an image information reading device as the above-described image reading device which reads a biomolecule distribution image from the gel support or transfer support as described above in which the biomolecules are two-dimensionally distributed.
The image information reading device is an optical head scanning type of image information reading device. When the image information reading device irradiates a sample with excitation light and guides fluorescence emitted from the sample to detection means by an optical head, scanning means causes the optical head to move at a constant velocity with respect to the sample.
If described in detail, a hole mirror having a hole in the center thereof is mounted on the optical head which moves in a main scanning direction. Organism-derived biomaterials marked by fluorochrome are distributed in a scanning target. Laser beam (excitation light) having a wavelength at which the fluorochrome is excited, emitted from a laser light source, is reflected upwardly from a planar mirror. The reflected laser beam passes through the small hole of the hole mirror, and then the laser beam is concentrated at an objective lens and applied to the scanning target. The fluorochrome in the scanning target is excited to emit fluorescence. The fluorescence is reflected around the small hole of the hole mirror. A photo multiplier performs photoelectric conversion of the fluorescence and the converted fluorescence is detected. In this manner, the detected fluorescence is read as a corresponding image signal. Hereinafter, it is assumed that the above-described operation is repeated while the optical head is moved in a sub-scanning direction orthogonal to the main scanning direction, and thus a two-dimensional visible image (fluorescence image) is obtained.
A surface plate is disposed and fixed on a lower side of a sample stand. The laser light source and the photo multiplier are mounted on a lower surface of the surface plate. The optical head, a driving device for moving the optical head in the main scanning direction, and a driving device for moving the optical head in the sub-scanning direction are mounted on an upper surface of the surface plate.
However, the image information reading device in the related art has a problem as described below.
That is, the laser light source and the photo multiplier are mounted on the lower surface of the surface plate, and the optical head, the driving device for moving the optical head in the main scanning direction of the optical head, and the driving device for moving the optical head in the sub-scanning direction are mounted on the upper surface of the surface plate. The laser beam from the laser light source is reflected from two mirrors and is incident to the optical head which is mounted on the upper surface of the surface plate and moves in a two-dimensional direction. The fluorescence from the optical head is reflected from three mirrors and is incident to the photo multiplier which is mounted on the lower surface of the surface plate.
Accordingly, high-sensitivity is expected, however, an optical path of the laser beam from the laser light source to the scanning target and an optical path of the fluorescence from the scanning target to the photo multiplier are long. If the entirety of the image information reading device expands thermally, an extension ratio of the optical paths increases. A position relationship of optical members such as a lens and a mirror is shifted from a designed value, and thus there is a problem in that optical characteristics are degraded.
The various lenses and mirrors being the optical members and a motor of the driving device being a heat source are put on both sides of the surface plate. Accordingly, there is a problem in that the optical members are easily affected by a change in temperature of the atmosphere due to the heat source. As the rotation number of the motor becomes large so as for the optical head to perform scanning at a high speed, the more the optical members are further affected.